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Re: New kernel interface for sys_tz and timewarp?
- From: Karel Zak <kzak at redhat dot com>
- To: Alexandre Belloni <alexandre dot belloni at bootlin dot com>
- Cc: Lennart Poettering <mzxreary at 0pointer dot de>, Arnd Bergmann <arnd at arndb dot de>, "Theodore Y. Ts'o" <tytso at mit dot edu>, Linus Torvalds <torvalds at linux-foundation dot org>, Linux Kernel Mailing List <linux-kernel at vger dot kernel dot org>, Thomas Gleixner <tglx at linutronix dot de>, John Stultz <john dot stultz at linaro dot org>, Stephen Boyd <sboyd at kernel dot org>, Florian Weimer <fweimer at redhat dot com>, "H. Peter Anvin" <hpa at zytor dot com>, Palmer Dabbelt <palmer at dabbelt dot com>, Alistair Francis <alistair dot francis at wdc dot com>, GNU C Library <libc-alpha at sourceware dot org>, OGAWA Hirofumi <hirofumi at mail dot parknet dot co dot jp>
- Date: Mon, 19 Aug 2019 13:09:03 +0200
- Subject: Re: New kernel interface for sys_tz and timewarp?
- References: <CAK8P3a0VxM1BkjY1D2FfHi6L-ho_NH3v3+gBu45EfpjLF5NU5w@mail.gmail.com> <CAHk-=wiO2CWONDBud4nxoPgUJN1JEewFWhHa5wAqY8G5rrTXRQ@mail.gmail.com> <20190814000622.GB20365@mit.edu> <CAK8P3a1CXRETxn6Gh_cOxM3rZ-wUwVDu-7=yEwjqOY=uEdC6OQ@mail.gmail.com> <20190814090936.GB10516@gardel-login> <20190814093208.GG3600@piout.net>
On Wed, Aug 14, 2019 at 11:32:08AM +0200, Alexandre Belloni wrote:
> On 14/08/2019 11:09:36+0200, Lennart Poettering wrote:
> > On Mi, 14.08.19 10:31, Arnd Bergmann (arnd@arndb.de) wrote:
> >
> > > - glibc stops passing the caller timezone argument to the kernel
> > > - the distro kernel disables CONFIG_RTC_HCTOSYS,
> > > CONFIG_RTC_SYSTOHC and CONFIG_GENERIC_CMOS_UPDATE
> >
> > What's the benefit of letting userspace do this? It sounds a lot more
> > fragile to leave this syncing to userspace if the kernel can do this
> > trivially on its own.
Good point, why CONFIG_RTC_SYSTOHC has been added to the kernel?
If I good remember than it's because synchronize userspace hwclock
with rtc is pretty fragile and frustrating. We have improved this
hwclock code many times and it will never be perfect. See for example
hwclock --delay= option, sometimes hwclock has no clue about RTC behaviour.
> It does it trivially and badly:
>
> - hctosys will always think the RTC is in UTC so if the RTC is in
> local time, you will anyway have up to 12 hours difference until
> userspace fixes that.
Cannot we provide all necessary information for example on kernel
command line, or/and as rtc module option?
Karel
--
Karel Zak <kzak@redhat.com>
http://karelzak.blogspot.com